Process and apparatus for recovering sterilization gas for reuse



R. R. ERNST Dec. 22, 1970 PROCESS AND APPARATUS FOR RECOVERINGSTERILIZING` GAS FOR REUSE Filed May 6, 1968 Mm @QM ATTORNEY UnitedStates Patent O 3,549,312 PROCESS AND APPARATUS FOR RECOVERINGSTERILIZATION GAS FOR REUSE Robert R. Ernst, Rochester, N.Y., assignorto Sybron Corporation, a corporation of New York Filed May 6, 1968, Ser.No. 726,974 Int. Cl. A611 3/00, 13/ 00 U.S. Cl. 21-58 9 Claims ABSTRACTOF THE DISCLOSURE A process and apparatus for sterilizing an object (eg.material and/ or equipment) with a gaseous sterilizing agent wherein thegaseous sterilizing agent (e.g., an alkylene oxide) is recovered fromthe sterilizing chamber for reuse in subsequent sterilizing operations.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to vapor sterilization and in particular to a sterilizingprocess using volatile alkylene oxides which includes recovering thegaseous alkylene oxide and mixtures thereof for reuse in a subsequentsterilizing operation.

Description of the prior art The term sterilant as used in thisapplication means a sterilizing agent capable of completely inactivatingor destroying the viability of all types of micro-organisms and insectsand their eggs and larvae contained on or in 'inanimate objects. Avolatile sterilant is one that acts chieily in the vapor state.

Steam sterilization at high temperature is a conventional gassterilization treatment, but certain pieces of equipment that are usedin hospitals are deleteriously affected, or are attacked, by steam; andconsequently sterilization of these articles with steam is either notpractical, or not entirely satisfactory. For example, the high heat ofsteam sterilization attacks, and may destroy, materials such asplastics, rubber, waxes, certain adhesives, and many drugs. Then, too,the high moisture content of steam causes tarnishing, dulling, andrusting, of surgical instruments, knives and other metallic surgicalinstruments. Steam too, is not suitable for sterilizing such articles astemperature gauges that are affected by heat. In some instances, too, itis diflicult to make certain goods bacteriologically safe with steam,as, for instance, tubing for intravenous Work. Here the inside of thetubing may not be reached to the required degree Iby the steam.

For these reasons, in recent years attempts have been made to developprocesses and equipment for effective sterilization through the use ofnon-corrosive gases that are effective at ordinary temperatures or attemperatures below the temperature at which steam is effective.

Ethylene oxide has many desirable characteristics as a sterilizingagent. For example, it is non-corrosive and does not damage thesubstance or equipment being sterilized. It is usable at lowtemperature. Moreover, it is destructive to all forms of organisms; andit has a relatively rapid action. Furthermore, after it has been used,it can be removed by aerating the sterile material.

However, the use of ethylene oxide as a sterilizing agent is subject toseveral disadvantages. For example, ethylene oxide is quite flammableand explosive when mixed with air in certain proportions. Some measureof relief has been obtained from the explosive and flammabilitycharacteristics of ethylene oxide by diluting it with carbon dioxide,Freons, Ucons, or chemically similar inert compounds, to render themixture inert, and storing ICC it as a liquid solution under highpressure and/or low temperature. Other non-flammable, mutuallynon-reactive diluents are known for combining with ethylene oxide suchthat the mixture is non-flammable and non-explosive. One of theprincipal disadvantages of using inerted ethylene oxide as a vaporsterilizing agent is its high cost.

It is a primary object of the present invention to reduce the cost ofsterilizing processes using a gaseous alkylene oxide as the vaporsterilizing agent.

It is another object of the present invention to provide a sterilizingprocess which uses gaseous ethylene oxide and mixtures thereof as thesterilizing agent and which recovers such mixtures for reuse in thesterilizing process.

It is a further object of the present invention to remove the sterilantmixture from a sterilizer, to separate air and moisture from it, and tocondense it for reuse.

SUMMARY OF THE PRESENT INVENTION In the process of sterilizing materialand equipment to produce biocidal action using volatile alkylene oxidesterilants which have preferably been rendered non-Hammable andnon-explosive, the improvement comprising removing the gaseoussterilizing mixture from the sterilizing chamber, separating air andmoisture from the mixture, condensing it to the liquid state, andstoring the recovered sterilant mixture in a liquid sterilant reservoirfor reuse in the process whereby the cost of the process is reduced.

BRIEF DESCRIPTION OF THE DRAWING These and other objects and advantagesof the present lnvention will be more fully understood by reference tothe attached drawing in which like reference numerals refer to likeelements, and in which:

The figure is a schematic iiow diagram of the process and apparatus ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Conventional gassterilization treatment processes wherein the used sterilant mixture isdischarged into the atmosphere are old and well-known as disclosed forexample in U.S. Pat. No. 3,068,064, issued Dec. 11, 1962, to Robert L.McDonald, and assigned to the same assignee as 1s the subjectapplication. Such processes therefore need not be described in detailhere.

The figure illustrates the preferred embodiment of the present inventionwherein a sterilant mixture comprising 12% by weight ethylene oxide inFreon 12 dichlorodifluoromethane is removed from a sterilizing chamber10, condensed, and fed to a sterilant supply reservoir 12 for reuse inthe sterilizing chamber 10. An object (such as some material or a pieceof equipment) is positioned in the chamber 10 and sterilized bysupplying a gaseous sterilizing agent thereto from the reservoir 12through a valve 13, 'feed line 14, a gas metering unit 16, a heatexchanger 18, and a valve 20, as is well known in the art. However,according to the prior art the gaseous sterilant mixture when thenpumped out and discharged to atmosphere from chamber 10 through, forexample, a pump 21 and a valve 22 before opening up the chamber toremove the sterilized subject. As described above this is an expensiveprocedure because of the total loss of the expensive sterilant mixture.

According to the present invention this disadvantage is eliminated byrecovering the gaseous sterilizing agent, which can be pure sterilizinggas but which is preferably a sterilant mixture, from chamber 10 forreuse in subsequent sterilizing operations. This is accomplished byopening a valve 23 in a line 26 connected to the bottom of the chamber10 to feed the gaseous sterilizing agent out of chamber 10 to a cooler24 by means of a pump 28.

Alternatively the pump can be positioned as a vacuum pump to exhaust thecondenser (infra). At the same time that Valve 23 is opened (valve 20 isof course closed), a vent valve 30 connected to vent line 31 adjacentthe top of chamber is preferably opened to replace the gaseoussterilizing agent which has been removed from `the sterilizer 10 throughline26 with air or some other lower density gas; a filter is preferablypositioned in line 31 to allow for the admission of sterile air. Thegaseous sterilizing agent is effectively removed from chamber 10 bytaking advantage of the fact that it hasa density about four timesgreater than that of air whereby it-can be pumped out of the sterilizingchamber 10 near the bottom thereof While air is allowed to enter at thetop thereof without any substantial mixing therebetween due to thestratification that occurs between the two different density gas phases.

The gaseous sterilant mixture flows from the preliminary cooler 24 to adrier 32 where excess moisture is removed therefrom. The dryer can beone ofthe type which uses a moisture absorbent material such as calciumchloride arranged, for example, in a column; this type of dryer alsoprovides `a visual color change when the effectiveness of the materialhas terminated. Since the primary function of the cooler is to condenseout moisture, which function is also served by the dryer, it is possibleto eliminate the cooler from the system. The gas then flows to a maincondenser 36 (flow rate being measured by means of a rotometer 34),where it is liquified by means of a plurality of cooling coils suppliedwith a refrigerant from a refrigeration unit 38. A non-return valve (notshown) `is Vpreferablypositioned downstream from the rotometer 34 to`prevent ow from the line 44 to the rotometer 34. Connected to thebottom of the main condenser 36 is a collecting tank 40 for collectingthe liquified sterilizing agent which drains by gravity into the tank. r.t

Itfshould be notedthat it is not absolutely necessary tovhave a pump tomove the gases vsince the condensing process will keep the gasesflowing. However, if airis present in the system a vacuum pump can beused to remove air from the condenser. The condenser will have residualsterilant mixture vapor therein at all times and possibly will remainunderfvacuum `when the refrigeration unit ison. The condenser willprobably befunder ethylene oxide- Freon-12 pressure when therefrigeration unit` is off. The air in the condenser will be displacedby the sterilant mixture and will be removed by means of Va pressurecontrolled safety valve 41 connectedto the condenser 36 the reservoir12. At this timea valve 42 in a pressure equalizing line 44 and a valve46 in a-drain line-47 are opened. At this time the liquid which has beencollected in tank 40 will drain into the reservoir 12. As shown in theligure, the drain line 47 is connected to thepressure equalizer line 44for draining liquidfrom the tank 40 into thereservoir 12, however, thedrain line 47 can alternatifely be connected directly to the reservoir12.

A float control 48 maintains the liquid level in the'reservoir 12 atapredetermined height. If the liquid level falls below said predeterminedheight the float control 48 will open a solenoid control valve 50whereby a fresh supply of the liquid sterilant mixture enters thereservoir 12 from a sterilant mixture supply cylinder 52 via line 54.The sterilant mixture in cylinder52 at room temperature (75 F.) isgundera pressure of about 70 p.s.i.g. The sterilant mixture thus feeds fromthe. cylinder 52 to the reservoir 12 under its own pressure when thepressure in 75 thereservoir 12 is reduced, such as when the operatingtemperature of the reservoir 12 is below room temperature due to the lowtemperature of the liquid returning to the reservoir 12 from the maincondenser 36, and/or when the reservoir 12 pressure is otherwisereduced, such `as when the sterilizer 10 is being filled. The valve 50is preferably a unidirectional valve permitting flow only from thecylinder 52 to the reservoir 12. Cylinder 52 is provided with aninternal tube which opens adjacent the bottom of the cylinder 52.

Also connected to the reservoir 12 is an inert gas (in the preferredembodiment, Freon-12) supply cylinder 56, `by means of a line 58 and apressure controlled solenoid valve 60. The inert gas is also under itsown vapor pressure and it feeds as a liquid to reservoir 12 when thevapor pressure in the reservoir falls below a pre-set level. The vaporpressure of Freon-12 is always higher, at the same temperature, thanthat of the mixture in the reservoid. When the vapor pressure in thereservoir falls below an expected pressure level, for example for a 10%ethylene oxide-% Freon-l2 mixture,which it is desired to maintain in thereservoir, this indicates an ethylene oxide rich mixture. The pre-setpressure controlled switch 60 at such time places the Freon-12 make-upsupply cylinder on demand until the Vapor pressure in the reservoirincreases up to the desired range. The make-up supply cylinder 56 helpsto maintain the proper mixture range of inerted ethylene oxide andprevents flammable mixtures from occurring.

In addition to the above described connection to reservoir 12 of themake-up supply cylinder 56 through pres- `sure controlled valve 60, thereservoir also is provided with a line 61 and a solenoid,pressurecontrol vent valve 62 which is set to open at a predeterminedpressure. The valve 60 determines the lower end and valve 62 determinesthe upper end of the pressure range under which the vapor in thereservoir is maintained. Inv the preferred embodiment (l0-12% mixture ofethylene oxide in Freon- 12), this pressure range is below 70 p.s.i.g.

In the preferred embodiment the commercial mixture contained in thesterilant mixture supply cylinder 52 is a 12% by Weight mixture (27.3%molar) of ethylene oxide in Freon-12 (dichlorofluoromethane). A 10-l2%mixture can be maintained in reservoir 12 from the 12% supply cylinder.By keeping the solenoid, pressure control vent valve 62 set to open at apredetermined pressure, and by maintaining the temperature in thereservoir 12 within well defined limits (70 F. i5 F. in this embodiment)the liquid in the reservoircan be kept within the 10-l2% ethylene oxiderange. The sterilizing agent can then be supplied to the sterilizer 10directly from the reservoir 12 upon demand. The efficiency of thissystem is estimated to bein excess of 90%. The temperature can bemaintained by means of a temperature controller 63, such. as a `waterjacket or externalstrip heaters with insulation. The valve 62 ventspre-condensed air and serves as a safety valve. Also, the object(s) (eg.material and/or equipment) being sterilized may preferentially absorbethylene oxide thus leaving excess Frech-12 to be condensed.Subsequently, the excess Freon-12 will provide an excessive vaporpressure, above that expected for the designated 12% mixture. Thus, thevalve 62 will open under the excess pressure and vent the excessFreon-12.

The process of the subject invention is applicable for use withsterilizing agents such asi ethylene oxide and inerted ethylene oxideand with otherV sterilants than ethylene oxide; for example, otheralkylene oxide sterilants such as propylene oxide can also be used. Theused sterilizing agent which is recovered for` reuse according to thepresent invention is preferably condensed back to its liquid state butcan be recovered and reused, all while still in the gas state. Theprocess is preferably a batch process but a continuous process ofsterilizing, recovering, and reusing is also possible. The system of thepresent invention reduces the high cost usually associated with gaseoussterilization processes. ln the case of ethylene oxide, the presentinvention makes the use of the safe non-hazardous mixtures thereofcomparable in cost to the use of the less expensive explosive, flammable100% ethylene oxide. This is especially true since the use of theammable explosive gases require higher insurance costs and very highcapital cost for explosion-proof components and isolated, make-safequarters. V

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention as described hereinabove and as defined in the appendedclaims.

I claim: 1. A process for sterilizing an object with a gaseoussterilizing agent comprising an inerted mixture of alkylene oxide, saidprocess comprising:

exposing an object to be sterilized to said gaseous sterilizing agent,

recovering said gaseous sterilizing agent for reuse, and

condensing said recovered sterilizing agent to its liquid state.

2. A process for sterilizing an object with a gaseous sterilizing agentcomprising an inerted mixture of ethylene oxide, said processcomprising:

exposing an object to be sterilized to said gaseous sterilizing agent,

recovering said gaseous sterilizing agent for reuse, and

condensing said recovered sterilizing agent to its liquid state. p

3. The process according to claim 2 including the step of:

feeding the condensed, liquid sterilizing agent to a liquid sterilizingagent supply reservoir.

4. The process according to claim 3 including the step of:

controlling the percentage range of said ethylene oxide in said inertedmixture.

5. The process according to claim 4 wherein said controlling stepcomprises:

introducing a fresh supply of said inerted sterilizing mixture to saidreservoir when the liquid level in said reservoir falls below apredetermined level, and introducing into said reservoir, when the vaporpressure in said reservoir falls below a predetermined value, suflicientgas to raise said pressure back up to another predetermined Value, saidintroduced gas comprising the inert gas of said inerted mixture.

6. In a process for sterilizing an object with gaseous ethylene oxide,the improvement comprising:

recovering said ethylene oxide for reuse,

condensing the recovered ethylene oxide to its liquid state, and

feeding the condensed ethylene oxide to a supply reservoir.

7. A process for sterilizing an object with a sterilizing agent in itsvapor state comprising an inerted mixture of ethylene oxide comprisingthe steps of:

storing said sterilizing agent in the liquid state in a supplyreservoir;

vaporizing a quantity of said sterilizing agent;

sterilizing an object with said vaporized sterilizing agent;

condensing the resulting used vaporized sterilizing agent back into itsliquid state; and

returning said condensed sterilizing agent to said reservoir.

8. A vapor sterilizing apparatus comprising:

a sterilizing chamber;

a reservoir adapted to contain a liquid sterilizing agent comprising amixture of a sterilant in an inert substance in a predeterminedpercentage range of sterilant to inert substance;

means for feeding said sterilizing agent from said reservoir to saidsterilizing chamber in its vapor state including means for vaporizingsaid liquid sterilizing agent;

means for removing said vaporous sterilizing agent from said sterilizingchamber;

means for condensing said vaporous sterilizing agent removed from saidchamber back into its liquid state;

means for feeding said condensed sterilizing agent to said reservoir;and

means for maintaining said mixture in said reservoir at substantiallysaid predetermined percentage range.

9. The apparatus according to claim 8 wherein said maintaining meanscomprises:

means for maintaining the temperature of said reservoir within apredetermined temperature range, and means for maintaining the pressurewithin said reservoir within a predetermined pressure range.

References Cited UNITED STATES PATENTS 2,131,134 9/1938 Baer et al.21-58 2,526,974 10/ 1950 Schiponski 21-94UX 3,042,533 7/ 1962 McConnellet al. 21-58X 3,068,064 11/ 1962 McDonald 21-58 3,088,179 5/1963Leuthner 1 21--58X 3,107,975 10/1963 Linder 21-94 3,341,280 9/1967Bolkin 21-58 3,361,517 1/1968 Skaller 21-94X 3,372,980 3/1968 Satas21-58 MORRIS O. WOLK, Primary Examiner B. S. RICHMAN, Assistant Examiner

